The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a system and method for multi-echo bandwidth matched imaging processes.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt.
The practical value of this phenomenon resides in the signal, which is emitted by the excited spins after the excitation signal B1 is terminated. In simple systems, the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A0, is determined by the magnitude of the transverse magnetic moment Mt. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
The amplitude, A, of the emission signal decays in an exponential fashion with time, t. The decay constant 1/T*2 depends on the homogeneity of the magnetic field and on T2, which is referred to as the “spin-spin relaxation” constant, or the “transverse relaxation” constant. The T2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B1 in a perfectly homogeneous field. The practical value of the T2 constant is that tissues have different T2 values and this can be exploited as a means of enhancing the contrast between such tissues.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process that is characterized by the time constant T1. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T1 time constant is longer than T2, much longer in most substances of medical interest. As with the T2 constant, the difference in T1 between tissues can be exploited to provide image contrast.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region that is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles that vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) that have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
The time required to acquire sufficient NMR signals to reconstruct an image is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences that have a very short repetition time (TR) and result in complete scans that can be conducted in seconds rather than minutes.
The concept of acquiring NMR imaging data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C. 10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a series of gradient-recalled NMR echo signals for each RF excitation pulse. These NMR signals are separately phase encoded so that a set of views sufficient to reconstruct an image can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging (“EPI”) are well known.
A variant of the echo-planar imaging method is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled “RARE Imaging: A Fast Imaging Method for Clinical MR.” The primary difference between the RARE sequence and the EPI sequence lies in the manner in which NMR echo signals are produced. The RARE sequence, utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill sequence, while EPI methods employ gradient recalled echoes.
Both of these “fast spin echo” imaging methods involve the acquisition of multiple echo signals from a single excitation pulse in which each acquired echo signal is separately phase encoded. Each pulse sequence, or “shot”, therefore results in the acquisition of a plurality of views and single shot scans are commonly employed with the EPI method. However, a plurality of shots is typically employed to acquire a complete set of image data when the RARE fast spin echo sequence is employed. For example, a RARE pulse sequence might acquire 8 or 16 separate echo signals, per shot, and an image requiring 256 views would, therefore, require 32 or 16 shots, respectively.
Pulse sequences based on spin echo, RARE, and EPI often employ pulse sequences with a preparatory pulse followed by a time delay prior to the imaging pulse sequence RF excitation. One such pulse sequence is referred to as an inversion recovery (IR) pulse sequence. The time delay between the inversion RF pulse and the RF excitation pulse is referred to as the inversion time (TI). Conceptually, an IR pulse sequence includes a first portion, referred to as the “IR module,” that includes the preparatory pulse, an optional spoiler gradient, and any slice-selection gradient (should the preparatory pulse be selective). The second portion of the IR pulse sequence, referred to as the “host sequence,” begins after the TI interval and typically includes a self-contained pulse sequence, such as a spin-echo sequence, gradient echo sequence, RARE sequence, EPI sequence, or the like.
Spin echo, RARE, and EPI pulse sequences often include an IR module for each host sequence. However, when fast gradient echo sequences are employed, the short TR does not allow time for a full IR module to be included before each imaging pulse sequence. As described by J. P. Mugler et al in “Three-Dimensional Magnetization-Prepared Rapid Gradient-Echo Imaging (3D MP RAGE),” Magnetic Resonance In Medicine 15, 152-157 (1990); by M. Brant-Zawadzki in “MP RAGE: A Three-Dimensional, T1-Weighted, Gradient-Echo Sequence—Initial Experience in the Brain,” Radiology 1992; 182: 769-775; and by J. P. Mugler et al. in “T2-Weighted Three-Dimensional MP-RAGE MR Imaging,” JMRI 1991: 1:731-737; a plurality of gradient-echo pulse sequence can be performed after each IR module. In particular, for T1-weighted imaging, a non-selective preparatory pulse (having an angle selected from 0 to 180 degrees) is applied and followed by a TI interval. After the TI interval, a series of fast gradient-recalled echo sequences are performed to acquire a corresponding series of phase-encoded lines in k-space. Following a recovery period, the process is repeated as necessary to fully sample k-space.
A common practice when using MR images in a clinical setting is to register and combine images produced using different pulse sequences. These different images each provide different tissue contrasts and their combination provides the necessary tissue contrast between all of the clinically important tissue types in the volume of interest. In multi-spectral brain morphometry, for example, an MP-RAGE image, a multi-echo FLASH image, and a fast spin echo (FSE) image may be acquired and registered with each other to examine the edges of small structures in the brain. Due to B0 field inhomogeneities in the MRI system, however, all of these different images should be acquired at the same signal readout bandwidth so that distortions due to the B0 inhomogeneity will be identical in all the images and they can be precisely registered with each other. A limitation of the MP-RAGE pulse sequence is that when its readout bandwidth is increased to match those of other pulse sequences, the signal-to-noise ratio (“SNR”) of the acquired NMR echo signal is reduced to an unacceptable low level.